Keycap
A keycap is a small piece of plastic (or occasionally another material) that sits on top of a switch. Keycaps are the parts of a mechanical keyboard that you actually touch when you type.
Unless it's barebones, the keyboard should come with all its switches and keycaps already installed. If it's hot-swappable, you can swap these out for any you prefer.
Keycaps are the most prominent part of the keyboard, that you see and feel. Much like clothes, many designs are available to reflect your personality.
When perusing keycaps, the most important thing you need to make sure of is that they're compatible with your switches. With the exception of low profile keyboards, Cherry MX switches and their clones are the de facto standard.
You should also make sure they're compatible with whichever layout you're using, ANSI or ISO.
Keycaps come in a variety of subtly different shapes known as their profiles; at least two main materials; and most obviously to look at, different colours and legend styles. These are all a matter of personal preference.
Mechanical keyboards: Keycap | Keychron keyboards | Southpaw | Switch (mechanical keyboard)
